It is known to provide a slider window assembly for an opening of a vehicle, such as a rear slider window assembly for a rear opening of a pickup truck. Conventional slider window assemblies for rear openings of trucks or the like typically include three or more panels, such as two fixed window panels and a slidable window panel. The slidable window panel is supported by rails and may be moved along the rails to open and close the window. The slidable window panel may be driven or moved by a cable drive system, with a cable sheath received at end caps that are attached at end surfaces of a generally U-shaped guide rail, such as described in U.S. Pat. No. 8,151,519, which is hereby incorporated herein by reference in its entirety. Such end caps are attached to and located partially within the respective ends of the generally U-shaped guide rail, with the end caps each including an abutment flange that engages the end surface of the generally U-shaped guide rail when the end cap is fixedly attached at the end of the rail.